I've been riding fixed for 3 years. This past year I road Cycle North Carolina. This starts in the mountains and ends on the coast after about 500 miles. I used a flip-flop with 2 available gear ratios and did fine. The worst parts were the long decents on days 1 and 2 and the unrelenting 15 mph headwinds toward the end.
But around home I am surrounded by elevation changes that are 1/2 mile long, give or take, and 200 feet, give or take. The problem is that the climbs are pitchy. It is not unusual for them to kick up to 15-20% in spots, and when that happens I just can't make it. So, I have to pick routes around climbs I can make.
Is the S3X my answer?